The image-sensor picture elements (pixels) known from literature are either of the photodiode or the MOS device type, as described for example in P. Seitz, “Solid-State Image Sensing”, in Computer Vision and Applications—A Guide for Students and Practitioners (B. Jähne and H. Haussecker, Eds.), pp. 111-152, Academic Press, San Diego, 2000. In such photo sensors, the photo-charge detection sensitivity varies with the inverse of the total capacitance of the pixel. This capacitance, on the other hand, increases monotonously (for larger pixels almost linearly) with the pixel size. For this reason, it is not possible to realize such conventional photo sensors that are at the same time very large and highly sensitive.
A first method that allows the realization of two-dimensional arrays of large-area and highly sensitive pixels is described by P. Seitz et al. in “Smart optical and image sensors fabricated with industrial CMOS/CCD semiconductor processes”, Proc. SPIE, Vol. 1900, 21-30, 1993. The so-called “charge binning” method makes use of standard CCD technology and employs a special charge transport/accumulation technique. The CCD gates are clocked such that charge packets from different pixels are accumulated under one gate, so that this summed charge can be read out instead of reading out all pixel charge packets individually. In a two-dimensional CCD image sensor it is possible to employ this charge binning method to realize two-dimensional areas of an effective photosensitive size (“super pixels”) that are much larger than the areas of individual pixels, and these super pixels can even have non-rectangular shape. However, this implies the use of industry standard CCD technology for the fabrication of CCD image sensors, as well as suitable CCD clocking circuitry and schemes with the associated system complexity and high electrical power consumption. The charge transport process within the semiconductor bulk material that is typical for CCD structures, is primarily based on slow diffusion processes instead of fast drift processes, hence significantly limiting the response time.
U.S. Pat. No. 5,528,643 (J. Hynecek: “Charge coupled device/charge super sweep image system and method for making”) describes the fast lateral transport of photo-generated charge carriers by employing a series of CCD gates, each of which has contacts at both ends at which voltage differences can be applied. In this way, each CCD electrode exhibits a lateral drift field at the semiconductor-insulator interface. Thus, a two-dimensional CCD image sensor with improved photo-charge transport speed in the column and read-out line directions is created. As in the charge binning approach described above, this architecture necessitates CCD clocking circuitry and clocking schemes. Again, system complexity and power consumption will be rather high. Furthermore, no demodulation action for an incident modulated wave field can be obtained with such a device, since photo-charge can only be moved in one fixed direction.
A second method to overcome the contradiction of simultaneous implementation of large sensitive area and small conversion capacity is based on the idea of K. Hoffmann (Solid State Electronics, Vol. 20, 1977). Charge carriers are transported by drift fields rather than by a diffusion process over a large area with subsequent storage in a diffusion node with small conversion capacity. This fast and efficient charge-transport method was used first by K. Hoffmann in memory cells. J. Lohstroh proposed in U.S. Pat. No. 4,245,233 (Lohstroh: “Photosensitive device arrangement using a drift field charge transfer mechanism”) the exploitation of the method for the collection of photo-generated charges. An elongated one-dimensional MOS structure is described, consisting of a highly resistive layer on top of an insulator covering a semiconductor. A voltage difference is applied to the two ends of the highly resistive layer, creating in the corresponding spatial dimension a linearly increasing potential distribution at the interface between semiconductor and insulator. Charges are generated by incident radiation in the bulk of the semiconductor, and they move to the semiconductor-oxide interface essentially by diffusion. Once they are close to the semiconductor-oxide interface, they feel the spatially varying surface potential, and they move along the electric field lines to the region with lowest potential energy, at one end of the device. At this place, a diffusion implant at the semiconductor surface is employed to extract the photocurrent, making use of a transimpedance circuit that keeps the diffusion at a fixed potential. Since this type of photosensor makes use of one-dimensional elongated structures and a transimpedance circuit, the complete device covers a large area. Therefore, it is practically only useful for single photodetector sites or, at most, for a linear array of photodetector pixels that offer at the same time large areas and high charge detection sensitivity.
DE-44′40′613 C1 (Spirig, Seitz: “Vorrichtung und Verfahren zur Detektion und Demodulation eines intensitätsmodulierten Strahlungsfeldes”) teaches the detection and demodulation of intensity modulated wave fields with sensing elements that consist of three parts: one photosensitive part, in which incident photons are converted into a proportional number of electronic charge pairs, one or more storage elements, into which the photo-generated charges are stored and accumulated, and an equal number of switches between the photosensitive part and each storage element. The switches are operated synchronously with the modulation frequency. A preferred embodiment relies on charge coupled device (CCD) techniques, as described by A. J. P. Theuwissen in “Solid-state imaging with charge-coupled devices”, Kluwer, Dordrecht, 1995. There, the photosensitive site and the switches are realized and operated by CCD gates that enable the transport of the photo-generated charge laterally. Though the photo-sensitive area and the conversion capacity have been decoupled, allowing the combination of large area sensing and high sensitivity, the disadvantages of this approach include the limited demodulation speed that is obtained with CCDs, especially if large photosensitive sites and CCD gates are employed. Further disadvantages are the necessity of special semiconductor processes for the fabrication of the CCD structures, and the demands on clocking waveforms with specially shaped rising or falling edges in order to obtain high charge-transfer efficiency under the CCD gates. An alternate embodiment of the switches employs field effect transistors (FETs), as available in industry standard complementary metal oxide semiconductor (CMOS) processes. This type of switch is simpler to operate, and it is readily fabricated. The disadvantage of the FET switch is increased charge and voltage noise behavior due to incomplete charge transfer, charge injection effects and channel current noise caused by gate voltage fluctuations.
DE-198′21′974 A1 (Schwarte: “Vorrichtung und Verfahren zur Erfassung von Phase und Amplitude elektromagnetischer Wellen”) overcomes the speed limitations of large photosensitive elements by replacing the single large photo gate with a comb-like structure of interdigitated finger-electrode photo gates. The photo-generated charge carriers are therefore more rapidly collected, and they can also be transferred more quickly onto two or more storage elements. The teaching relies also on switching elements for transferring photo charge onto suitable storage elements. The disadvantages of these switching elements, realized as CCD gates of FETs, are the same as described for DE-44′40′613 C1.
EP-1′152′261 A1 (Lange, Seitz: “Device and method for spatially resolved photodetection and demodulation of modulated electromagnetic waves”) describes an alternative sensing element for the detection and demodulation of intensity-modulated wave fields. It employs two photo-sensing parts per sensing element, each with two storage sites and an associated switching element. When used in conjunction with a diffusing optical component on top of the sensing element for the equal distribution of the incoming wave field intensity on the two photo sites, this device allows prolonged integration times and relieves the timing restrictions on the clock waveform. The number of storage sites is limited to four, rendering this device ineffective if more than four samples per period of the modulated waveform should be taken. Since the teaching also relies on switches for the transfer of photo charges from the photo sites to the storage elements, the same disadvantages are encountered as described for the above two documents.
The basic photodiode and MOS device types and the enhanced pixel structures with separate photo-charge collection area and photo-charge storage and detection devices mentioned above have the disadvantage of one-dimensional pixel topology. As a direct consequence, in movements of photo-generated charges effectuated by drift fields, for example within the space charge region of a photodiode or between MOS devices (both with or without overlapping gate electrodes), there is very little control over the two-dimensional distribution of the electric field and, consequently, over the motion of the photo-charges. In addition, some of the pixel structures described above exhibit high power dissipation and/or low quantum efficiency. No known pixel device is capable of combining large-area sensing with high conversion gain, high quantum efficiency and in particular a two-dimensional spatial influence on the transport of the photo-generated charges by an optimized electric field distribution that can be fully controlled in the two lateral dimensions.